Modify Existing Scanning Electron Microscope (SEM)

SEM FE Probe Surface Science StudyL. Laurent, S. Tantawi, R. Kirby

*This research is funded by the SLAC LDRD Program

*Laboratory Directed Research and Development

SEM FE Probe Surface Science Study Overview

Field emission probe will detect features that exhibit premature field emission. The surface imaging capability of the SEM and elemental x-ray analysis can then be used to obtain topographical and material information about these emission sites.

In both cathode and high gradient research we are faced with challenging physics that will likely only be overcome by understanding the physics and chemistry that is taking place at the surface.

One of the strongest benefits of this system will be the ability to analyze not only test materials but actual components before and after they have been installed into systems.

SSRL Dispenser Cathode

Cathode Research

The performance of cathodes is highly dependent by the physics and chemistry taking place on the surface. The FE SEM system will be used to identify possible causes of non-uniform cathode emission.

LCLS Photocathode Inside SLAC SEM vacuum Chamber

The current probe will provide diagnostic support to help understand emission physics and in advancing next generation photocathode high brightness beams. We will use the current probe system to find features that may be responsible for breakdown.

Laser Cleaning

Breakdown

Cathode Research

Moly Plated Cu - Frascatti

CuZr-SLAC

CuCrCuZr-CERN

HIP Cu - KEK

TiN Coated Cu - KEK

CuZr-SLAC

W Iris Insert - CERN High Gradient Research

-- Materials --

High Gradient Research -- Surface Finish --

Pre-RF

Post-RF

Etch Pits

Electro-polished

Two Year Plan

Year 1:

1. Design and develop an electron microscope-based high-electric-field current-emission probe to study topographic material features and their impact on cathodes and high gradient structures.

2. Evaluate planar test samples in order to calibrate and optimize the system.

Year 2:

3. Work with non-planar geometries using existing components.

4. Perform iterative analysis on pre and post tested components (e.g. cathodes and high gradient structures).

LDRD SEM FE Probe Current Status and Program Overview -pg1

Design Stage:

Assembly drawings are completed. Field emission probe assembly and auxiliary components have been specified. Software control program outlined.

Ordering and Fabrication of Parts:

Fixturing for the probe assembly has been designed and fabricated. A data control and acquisition computer has been ordered has arrived (but still waiting

on GPIB/USB control cable). Keithley pico-ammeter has been received.

Waiting Delivery: Field Emission Probe (Capital equipment purchase order) has been delayed by the manufacturer and is now expected to arrive on May 27th.

Installation

The installation phase should be quick. We still need to drill a hole into the SEM vacuum chamber for a feedthrough flange.

We will not do this until FT flange arrives. No room for costly dimensional errors.

LDRD SEM FE Probe Current Status and Program Overview-pg2

Initial Testing and Calibration of the System on Planar Test Samples (e.g. Copper Coupons):

Software development phase.Begin test sample measurements and calibration by 6/30/11.

Analyze Existing (already tested) Cross Sectioned Accelerator Structures and Cathodes.

Evaluate working with non planar geometries.

Surface Science Study to Include Iterative Process (before and after rf testing and/or beam operation):

Cathode Research: Demountable Cathodes High Gradient Program: Clamped structures

New Higher Resolution SLAC SEM in 2011…??SLAC SEM is reaching end of life status. The goal is to optimize the system in SLAC’s current SEM and than move the system to a new SEM in the future when funding is available.

New SEM was approved in 2010 but purchase requisition was halted days before order was placed due to budget being on continuing resolution.

New RFP is being submitted within the month and we are hopeful that this one will go through and to have the new SEM before the end of this fiscal year (September 2011).

Present day SEM’s have significantly better capabilities and resolution than SLAC’s current SEM.

Summary

In high gradient structures and cathode development we are faced with challenging physics that will likely only be overcome by increasing our surface science capabilities.

The current probe coupled with SEM imaging and EDX spectrometry will provide additional diagnostic support to help understand surface and emission physics.

The SEM field emission probe will be available for a wide variety of applications.

We hope to increase the field emission probe system capabilities by installing it into a new higher resolution SEM as soon as funding is available.